In a known method of radio communication which is gaining acceptance for certain purposes, the energy in a radio frequency carrier is dispersed to occupy a relatively wide band of the RF spectrum. This type of communication has been designated by the terms "carrier dispersal" and "spread spectrum," the former referring to the process by which the energy associated with a carrier is dispersed or distributed over a relatively broad range of frequencies, and the term "spread spectrum" characterizing the waveform which results from this process. When the energy of the carrier is spread over a sufficiently wide frequency spectrum, its individual component frequencies become immersed in the background noise of the transmission channel, preventing the signal from being detected except by a selectively addressed receiver.
According to one known spread spectrum technique, the radio frequency signal is dispersed over a broad band of frequencies by modulating the carrier with a coded sequence of pulses derived from a pattern code generator. The outer limits to which the carrier bandwidth is spread in both directions from its basic frequency is f.sub.h which represents the highest frequency component in the modulation signal, and the individual frequencies which comprise the wideband having a spacing of f.sub.l, corresponding to the lowest frequency component of the modulation. An even spacing of the transmitted energy is achieved by providing these frequencies with a coded pulse modulation wherein the pulse width provides the bandspread desired, the repetition frequency of the code establishes the spacing between individual frequency components of the band, and the digit sequence of the code follows a random pattern with a statistically average distribution. Prior to spreading the carrier frequency of the transmitter, the transmitted energy is modulated by any of the known amplitude, frequency, phase, etc., modulation techniques to apply message intelligence to the carrier.
In the addressed receiver, a local code generator capable of generating the same code waveform as is used at the transmitter to disperse the energy in the carrier, modulates a local oscillator separated by the intermediate frequency (IF) bandwidth from the frequency of the transmitted carrier and beats the resultant output against the received signal in a correlation mixer. The output of the mixer is an IF signal containing only the information modulation provided the local code generator is in time synchronism with the received code signal modulation. The energy in the receiver is a maximum when the locally generated code is correlated with or in time synchronism with the received signal, and the energy level decreases if the receiver modulation leads or lags the incoming signal. Accordingly, the receiver necessarily requires a synchronizer to adjust the timing of the receiving pattern generator to maximize the energy in the receiver. The primary functions of the synchronizer are to compensate for timing errors between the transmitter and receiver code pattern generators and for changes in signal path distances which may occur due to variations in ionosphere or doppler velocities.
Synchronization between the transmitter and receiver, and maximum correlation to accomplish reassembly of the energy spread across the frequency spectrum back into a single carrier frequency, is facilitated by employing in both the transmitter and receiver a form of modulation which has a high auto-correlation function. Particularly useful for this purpose are the unique characteristics of the sequences of binary digits known as "maximum length shift register sequences" described in application Ser. No. 741,354 entitled "Selective Calling System" assigned to the assignee of the present application. The so-called "perfect word" outputs of this type of code generator comprise particular binary sequences of "zeros" and "ones" which, when correlated with shifted versions of themselves, provide maximum indication when they are aligned with exactly the same relationship of "one" and "zero" and a relatively minor correlation in all other shifted relationships. These perfect words also have the advantage, which will be referred to in more detail later, that they can, with the aid of suitable logic circuitry, be auto-generated to a sequence length of 2.sup.n -1 from an n-stage shift register.
Search for correlation between the transmitted code sequence and the one locally generated at the receiver may be accomplished by delaying the receiver pattern generator so that its code sequence, in effect, slides past the received code sequence in the correlation mixer of the receiver. When, during this sliding process, the two codes reach a point of precise identical digit alignment, all of their frequency and phase components become mutually additive and a relatively large signal appears in the IF amplifier. This signal applies a disabling voltage to the frequency shift circuit of the receiver code generator to restore the basic shift frequency and stop the search, or coarse synchronization, process. Once this coarse synchronism is attained, it is necessary in optimizing the output signal to lock the local code generator to the received signal. This process, which may be termed fine synchronization, is the problem to which the present invention is addressed.
Heretofore, fine synchronization in communication systems of this type has been accomplished by employing two receiver channels, to one of which the output of the local pattern generator is directly applied for local mixing, and to the other of which a time delayed version of the output of the pattern generator is applied. In operation, the basic shifting frequency of the pattern generator is shifted in time so as to increase the energy in the channel having the most energy. This form of synchronization has the disadvantage of requiring two mixers, two IF strips, two detectors and a comparison circuit. And, because two channels are used, the system is inherently subject to drifts in gain in the two channels, since a change in gain will shift the null of the synchronizer off the correlation peak, thereby introducing errors in the system.
Some of the disadvantages of the just-described synchronizing technique have been overcome in another previously used method by time-sharing between one mixer and one IF amplifier. The local code sequence is step advanced for a predetermined period and then step retarded for another period. As a result, the energy in the IF strip is amplitude modulated according to whether the advance step or the retard step results in more energy in the IF amplifier. This IF signal is detected to obtain the amplitude modulation, and the resulting signal is then phase detected, using the advance and retard signal as a reference, to obtain an error signal which is applied through a decision circuit to correct the time shift of the basic shifting frequency of the receiver code generator.
This approach to the fine synchronizing problem has the disadvantage that the bandwidth of the IF amplifier must be sufficiently wide to prevent a phase shift of the amplitude modulation. Increasing the bandwidth, of course, reduces the signal-to-noise ratio. If it is attempted to reduce the bandwidth to improve the signal-to-noise ratio, the leading and trailing edges of the square wave amplitude modulation (a square wave is necessary to the accuracy of the digital phase detectors used in the system) would be severely rounded off since the high frequencies attendent the sharp amplitude excursions would not be passed, with the result that maximum amplitude points would be shifted from their true time.
With an appreciation of the foregoing shortcomings of available synchronizing techniques in correlation communication systems, applicant has as a general object of the present invention to provide an improved means for maintaining close time synchronism, between remotely separated pulse code generators.
A more particular object of the invention is to provide a highly accurate means for maintaining time synchronism in a correlation communication system, wherein gain and phase drifts have little or no effect on the accuracy of the system.
Another object is to provide a synchronizing system for a signal correlation receiver which is capable of operation in a relatively narrow bandwidth to thereby improve the signal-to-noise ratio of the system.
Still another object of the invention is to provide a synchronizer for a signal correlation receiver whose accuracy is maintained over a wide range of received signal amplitudes.
Still another object of the invention is to provide a means for maintaining time synchronism in a correlation communications system receiver which is relatively simple to implement and which has long term stability.
Briefly, these and related objects are achieved by applying the IF output of a time shared correlation mixer through a phase reversing switch to a high "Q" resonant circuit. The phase reversing switch is synchronized with periodic phase advance and retard steps in the local oscillator modulating wave form. The phase of the energy in the high "Q" circuit at the end of each advance and retard cycle is compared in a phase detector with the phase of the energy in a second high "Q" circuit to which the intermediate frequency output of the correlation mixer is directly applied. The output of the phase detector is employed as a correction signal to change the basic clock frequency of the local pattern generator in a direction to effect perfect correlation with the received signal. The high "Q" circuits may be resonant filters of the integrate and dump type described in U.S. Pat. No. 3,056,890, arranged to be quenched at the end of each advance and retard cycle. The correction decision being based on phase information, saturating amplifiers may be employed to amplify the output signals from the high "Q" circuits prior to their application to the phase detector to ensure amplification of the smaller signals, and although the larger signals may be clipped, their phase is not destroyed. This feature gives the synchronizer a high degree of accuracy over a wide range of signal levels.